1. Field of the Invention
The present invention relates to a heat resistant layered porous silica used as catalyst carriers and as adsorbents of organic substances, etc. The present invention also relates to a process for producing the same.
2. Description of the Related Art
The catalysts for the catalytic cracking of petroleum and for the purification of exhaust gas are exposed to a relatively high temperature of 700.degree. C. or even higher. Accordingly, carriers for such catalysts are expected to maintain a large specific surface area even when they are brought under high temperatures, so that the active components might be kept well dispersed thereon.
However, the materials conventionally used as carriers such as zeolite, silica-alumina, alumina, and silica gel do not always exhibit a sufficiently high heat resistance, or at the expense of desirable pore size distribution, if any.
There is also a demand for an adsorbent having a high adsorption capacity used, e.g., as an absorber to be charged in automobile canisters for absorbing the evaporated fuel.
Recently, a porous silica having a uniform pore size distribution was synthesized (Bull. Chem. Soc. Jpn., 63 (1990), pp. 988-992). This type of porous silica can be obtained by expanding the interlayer spacing between the crystalline layered silicate.
The cross section of this porous silica yields a honeycomb porous structure, because the layered sheets of silica each take a finely waved structure, and the neighboring silica sheets partially develop siloxane bonding at bent portions thereof to form a three dimensional framework having fine pores being uniformly distributed in the structure.
The layered porous silica above can be produced by a process which comprises: synthesizing a crystalline layered silicate; dehydrating the crystalline layered silicate thus obtained; and subjecting the silicate to ion exchange using an organic cation and rinsing with water, thereby introducing the organic cation between the crystalline layered silicate sheets to expand the interlayer spacing and removing alkali metal ions such as sodium (Na.sup.+).
The process above, however, undergoes the aggregation of the crystalline layered silicate on dehydrating thereof. Then, the alkali metal ions cannot be sufficiently removed by the ion exchange treatment and rinsing with water. The residual alkali metal ions between the sheet layers reduce the specific surface area due to their crystallization into cristobalite or the like.
More specifically, the layered porous silica above described has a small specific surface area, 900 m.sup.2 /g at the most, and the specific surface area thereof considerably decreases in a temperature range of 800.degree. C. or higher. Therefore, the layered porous silica exhibits only such a poor heat resistance as illustrated in FIG. 3.